make privateAccess work with generic types and generic instantiations; fix a SIGSEGV (#18260)

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/stdlib/mimportutils.nim

@@ -13,5 +13,20 @@ type
     hd1: float
   PA* = ref A
   PtA* = ptr A
+  E*[T] = object
+    he1: int
+  FSub[T1, T2] = object
+    h3: T1
+    h4: T2
+  F*[T1, T2] = ref FSub[T1, T2]
+  G*[T] = ref E[T]
+  H3*[T] = object
+    h5: T
+  H2*[T] = H3[T]
+  H1*[T] = ref H2[T]
+  H*[T] = H1[T]
+
+type BAalias* = typeof(B.default)
+  # typeof is not a transparent abstraction, creates a `tyAlias`
 
 proc initB*(): B = B()

tests/stdlib/timportutils.nim

@@ -36,6 +36,55 @@ template main =
         let c = C(c0: 1, hc1: 2)
         c.hc1 == 2
 
+    block:
+      assertAll:
+        not compiles(E[int](he1: 1))
+        privateAccess E[int]
+        var e = E[int](he1: 1)
+        e.he1 == 1
+        e.he1 = 2
+        e.he1 == 2
+        e.he1 += 3
+        e.he1 == 5
+        # xxx caveat: this currently compiles but in future, we may want
+        # to make `privateAccess E[int]` only affect a specific instantiation;
+        # note that `privateAccess E` does work to cover all instantiations.
+        var e2 = E[float](he1: 1)
+
+    block:
+      assertAll:
+        not compiles(E[int](he1: 1))
+        privateAccess E
+        var e = E[int](he1: 1)
+        e.he1 == 1
+
+    block:
+      assertAll:
+        not compiles(F[int, int](h3: 1))
+        privateAccess F[int, int]
+        var e = F[int, int](h3: 1)
+        e.h3 == 1
+
+    block:
+      assertAll:
+        not compiles(F[int, int](h3: 1))
+        privateAccess F[int, int].default[].typeof
+        var e = F[int, int](h3: 1)
+        e.h3 == 1
+
+    block:
+      assertAll:
+        var a = G[int]()
+        var b = a.addr
+        privateAccess b.type
+        discard b.he1
+        discard b[][].he1
+
+    block:
+      assertAll:
+        privateAccess H[int]
+        var a = H[int](h5: 2)
+
     block:
       assertAll:
         privateAccess PA
@@ -44,6 +93,13 @@ template main =
         pa.ha1 = 3
         pa.ha1 == 3
 
+    block:
+      assertAll:
+        var b = BAalias()
+        not compiles(b.hb1)
+        privateAccess BAalias
+        discard b.hb1
+
     block:
       assertAll:
         var a = A(a0: 1)